Process for preparing a solution of lignin in an aqueous medium

ABSTRACT

The present invention relates to a process for lignin dissolution in which partial methylolation of lignin is carried out during the step of preparing a solution of lignin in an aqueous medium comprising alkali. The lignin prepared according to the process of the present invention can be used to manufacture lignin-based phenolic resins, which are particularly useful in the manufacture of laminates.

FIELD OF THE INVENTION

The present invention relates to a process for lignin dissolution inwhich partial methylolation of lignin is carried out during the step ofpreparing a solution of lignin in an aqueous medium comprising alkali.

The lignin prepared according to the process of the present inventioncan be used to manufacture lignin-based phenolic resins, which areparticularly useful for example in the manufacture of laminates,plywood, laminated veneer lumber and oriented strand board.

BACKGROUND

Lignin, an aromatic polymer is a major constituent in e.g. wood, beingthe most abundant carbon source on Earth second only to cellulose. Inrecent years, with development and commercialization of technologies toextract lignin in a highly purified, solid and particularized form fromthe pulp-making process, it has attracted significant attention as apossible renewable substitute to primarily aromatic chemical precursorscurrently sourced from the petrochemical industry.

Lignin, being a polyaromatic network has been extensively investigatedas a suitable substitute for phenol during production ofphenol-formaldehyde adhesives. These are used during manufacturing ofstructural wood products such as plywood, oriented strand board andfiberboard. During synthesis of such adhesives, phenol, partiallyreplaced by lignin, is reacted with formaldehyde in the presence ofeither basic or acidic catalyst to form a highly cross-linked aromaticresins termed novolacs (when utilizing acidic catalysts) or resoles(when utilizing basic catalysts). Currently, only limited amounts of thephenol can be replaced by lignin due to the lower reactivity of lignin.

Lignin may be utilized as a powder at the time that it is incorporatedinto the resin formulation. Lignin can also be utilized in “liquid form”in an alkali solution in order to avoid lignin dust. If the moisturecontent of the powder lignin is relatively low (0-5%), it can be dustyand may create respiratory hazards. If the moisture content of thesolids is relatively high (8-50%), it can be sticky or clumpy anddifficult to transfer in reliable and quantitative manner.

One problem when preparing resins comprising lignin is to ensure thatthe inherent reactivity of the lignin is fully utilized by adequatelydissolving it in a suitable liquid medium.

U.S. Pat. No. 4,113,675 is directed to a highly cross-linkablemethylolated kraft lignin resin for use as a high wet strength adhesive.

Malutan et al. (2008), BioResources 3(1), 13-20, is directed tohydroxymethylation of lignin and aims at determining reaction conditionsto improve the reactivity of lignin by fully methylolating lignin.

There is a need to facilitate the process for preparing a solution oflignin in an aqueous medium, particularly for preparation of resins.

SUMMARY OF THE INVENTION

It has now surprisingly been found that the solubility of lignin can beincreased by partial methylolation during the step of preparing asolution of lignin in an aqueous medium.

The present invention is thus directed to a method for dissolving ligninin an aqueous medium comprising the steps of

-   -   a) mixing lignin, alkali and formaldehyde in an aqueous medium;    -   b) mixing until a solution of lignin in the aqueous medium has        been obtained;    -   characterized in that the molar ratio between the formaldehyde        and lignin monomer is 0.4 or less.

Preferably, the method for dissolving lignin in an aqueous mediumcomprises the steps of

-   -   a) mixing lignin, alkali and formaldehyde in an aqueous medium;    -   b) mixing until a solution of lignin in the aqueous medium has        been obtained;

characterized in that the molar ratio between the formaldehyde andlignin monomer is 0.2 or less.

More preferably, the method for dissolving lignin in an aqueous mediumcomprises the steps of

-   -   a) mixing lignin, alkali and formaldehyde in an aqueous medium;    -   b) mixing until a solution of lignin in the aqueous medium has        been obtained;

characterized in that the molar ratio between the formaldehyde andlignin monomer is 0.2 or less and in that the lignin has been isolatedand/or purified before step a).

Most preferably, the lignin has been isolated from black liquor andoptionally purified before step a).

The present invention is also directed to a method for preparing aresin, comprising the steps of

-   -   a) mixing lignin, alkali and formaldehyde in an aqueous medium;    -   b) mixing until a solution of lignin in the aqueous medium has        been obtained;    -   c) keeping the mixture obtained in step b) at a temperature in        the range of from 30° C. to 95° C.; and    -   d) adding additional formaldehyde and phenol and maintaining the        mixture at a temperature of from 30° C. to 95° C. for at least        30 minutes;    -   characterized in that the molar ratio between the formaldehyde        and lignin monomer in step a) is 0.4 or less.

The present invention is thus also directed to resins and the use ofsaid resins in the manufacture of laminates, plywood, oriented strandboard (OSB), laminated veneer lumber (LVL), insulation and otherengineered wood products. The present invention is also directed to suchlaminates and engineered wood products manufactured using said resins.

The present invention is also directed to a method for increasing thesolubility of lignin in an aqueous medium comprising the steps of

-   -   a) mixing lignin, alkali and formaldehyde in an aqueous medium;    -   b) mixing until a solution of lignin in the aqueous medium has        been obtained;    -   characterized in that the molar ratio between the formaldehyde        and lignin monomer is 0.4 or less.

DETAILED DESCRIPTION

It is intended throughout the present description that the expression“lignin” embraces any kind of lignin, e.g. lignin originated fromhardwood, softwood or annular plants. Preferably the lignin is analkaline lignin generated in e.g. the Kraft process. Preferably, thelignin has been purified or isolated before being used in the processaccording to the present invention. The lignin may be isolated fromblack liquor and optionally be further purified before being used in theprocess according to the present invention. The purification istypically such that the purity of the lignin is at least 90%, preferablyat least 95%. Thus, the lignin used according to the method of thepresent invention preferably contains less than 10%, preferably lessthan 5% impurities. The lignin may be separated from the black liquor byusing the process disclosed in WO2006031175.

In one embodiment of the invention, an amount of alkali is added to theaqueous medium so that the aqueous medium comprises 5-50 wt % alkali,such as 10-50 wt % alkali, such as 40-50 wt % alkali. In one embodimentof the present invention the pH of the aqueous medium is at least pH 8,such as at least pH 10, such as at least pH 12 or at least pH13.

The alkali is preferably sodium hydroxide, potassium hydroxide, calciumhydroxide, magnesium hydroxide or a mixture thereof.

In one embodiment of the present invention, the molar ratio betweenformaldehyde and lignin monomer in step a) is 0.4 or less, such as 0.35or less or 0.30 or less or 0.25 or less or 0.20 or 0.15 or 0.10 or less.In one embodiment of the present invention the ratio betweenformaldehyde and lignin in step a) is at least 0.01, such as at least0.02 or at least 0.05. The molar ratio between formaldehyde and ligninis calculated based on an average lignin monomer molecular weight of 190g/mol. The molecular mass of the phenylpropane unit (C9) in lignin wasconsidered to be 190 g/mol. The duration of step c) of the processdescribed above is typically at least 5 minutes, such as at least 15minutes or at least 30 minutes. The temperature used in step c) is inthe range of 30-95° C., such as 40-80° C.

The mixing of lignin, alkali and formaldehyde in step a) can be carriedout in any order. Each component may for example be added consecutivelyor at the same time as one or two of the other components. In oneembodiment, the components are added in such a way that a part of thetotal amount of each component is added and one or more additionalamounts of each component is subsequently added.

It is preferred that the solution in step b) comprises 5-50 wt %, suchas 5-40 wt % lignin.

In one embodiment of the present invention, the mixing in step b) iscarried out at room temperature, such as at a temperature of from 15° C.to 25° C. In one embodiment of the present invention, the mixing in stepb) is carried out at elevated temperature, such as from 25° C. to 95°C., such as from 50° C. to 95° C.

The present invention also relates to a resin composition. The resincomposition is preferably a lignin-phenol-formaldehyde resin. Said resincomposition can be prepared by adding phenol and additional formaldehydeto the solution described above and heating said mixture. Thetemperature at the time of adding phenol and additional formaldehyde istypically in the range of from 40° C. to 60° C., such as about 50° C.Typically, the amount of phenol added is approximately the same as theamount of lignin, but it is appreciated that more or less phenol can beadded depending on what type of resin composition is desired. Theheating described in step d) is typically carried out at a temperatureof 70-85° C. for at least 30 minutes and typically for 4-8 hours.Typically, the viscosity increases during the heating in step d).

The resin obtained is useful for example in the manufacture oflaminates. The resin is then impregnated into and/or applied between thesheets that should form the laminate and said sheets are pressedtogether and heated at a temperature of about 130-150° C.

There are a number of advantages of the process described above,including:

-   -   increased solubility of lignin in the aqueous medium    -   shorter gel time of a resin obtained using lignin that has been        subjected to the process according to the present invention.

The present invention also relates to the use of the resin compositionin engineered wood products such as plywood, particle board, waferboard, gluelam beams, structural composite lumber, oriented strand board(OSB), oriented strand lumber (OSL), laminated veneer lumber (LVL) andother applications such as laminates, insulation and molding compounds.

EXAMPLES Example 1

Lignin based phenolic resin was synthesized for plywood applications. Inthe first step, lignin solution was prepared by mixing of 500 g of kraftlignin (solid content 95%), 961 g of water, 30.6 g of formalin(concentration 52.5%) and 194 g of 50% sodium hydroxide solution in a 5liter glass reactor equipped with overhead stirrer, condenser andtemperature control unit. Lignin solution was heated to 80° C. andcontinued heating at 80 ° C. for one hour. Then, the temperature of thelignin solution was decreased to 45° C.

In the second step, 484.8 g of phenol, 785.3 g of formalin(concentration 52.5%), 30 g of water and 52 g of 50% sodium hydroxidesolution were added to the lignin solution. The temperature wasincreased to 80° C. and 185 g of 50% sodium hydroxide solution was addedto the reaction mixture after 110 minutes of reaction. The reactionmixture was continuously heated at the temperature at 80° C. for further130 minutes and then cooled down to room temperature. The reaction wasmonitored by measuring the viscosity using the Brookfield DV-II+LVviscometer.

The gel time of the resin was measured by adding 10 g of the resin in aglass tube with a stamper made of aluminium. The glass tube was heatedat 100° C. in an oil bath. The stamper was moving up and down until theresin became gel. The time until the resin became gel was measured asgel time.

The resin was analyzed and the results of the analysis are given inTable 1.

TABLE 1 Resin Properties S.C (%) 45.4 pH 11.8 Gel time (min) @100° C. 50

Example 2

Lignin based phenolic resin was synthesized for plywood applications. Inthe first step, lignin solution was prepared by mixing of 500 g of kraftlignin (solid content 95%), 961 g of water, 73 g of formalin(concentration 52.5%) and 194 g of 50% sodium hydroxide solution in a 5liter glass reactor equipped with overhead stirrer, condenser andtemperature control unit. Lignin solution was heated to 80° C. andcontinued heating at 80° C. for one hour. Then, the temperature of thelignin solution was decreased to 45° C.

In the second step, 484.8 g of phenol, 743 g of formalin (concentration52.5%), 30 g of water and 52 g of 50% sodium hydroxide solution wereadded to the lignin solution. The temperature was increased to 80 ° C.and 185 g of 50% sodium hydroxide solution was added to the reactionmixture after 90 minutes. The reaction mixture was continuously heatedat the temperature at 80° C. for further 110 minutes and then cooleddown to room temperature. The reaction was monitored by measuring theviscosity using the Brookfield DV-II+LV viscometer.

The resin was analyzed and the results of the analysis are given inTable 2.

TABLE 2 Resin Properties S.C (%) 45.7 pH 11.6 Gel time (min) @100° C. 49

Example 3

Lignin based phenolic resin was synthesized for plywood applications. Inthe first step, lignin solution was prepared by mixing of 167 g of kraftlignin (solid content 95%), 315 g of water, 24.3 g of formalin(concentration 52.5%) and 82 g of 50% sodium hydroxide solution in a 1liter glass reactor equipped with overhead stirrer, condenser andtemperature control unit. Lignin solution was heated to 60° C. andcontinued heating at 60° C. for one hour. Then, the temperature of thelignin solution was decreased to 45° C.

In the second step, 161 g of phenol, 248 g of formalin (concentration52.5%) and 10 g of water were added to the lignin solution. Thetemperature was increased to 80° C. and 62 g of 50% sodium hydroxidesolution was added to the reaction mixture after 90 minutes. Thereaction mixture was continuously heated at the temperature at 80° C.for further 120 minutes and then cooled down to room temperature. Thereaction was monitored by measuring the viscosity using the BrookfieldDV-II+LV viscometer.

The resin was analyzed and the results of the analysis are given inTable 3.

TABLE 3 Resin Properties S.C (%) 46.8 pH 11.3 Gel time (min) @100° C. 48

Example 4

Lignin based phenolic resin was synthesized for laminate applications.In the first step, lignin solution was prepared by mixing of 117 g ofkraft lignin (solid content 95%), 215 g of water, 5.7 g of formalin(concentration 52.5%) and 26.1 g of 50% sodium hydroxide solution in a 1liter glass reactor equipped with overhead stirrer, condenser andtemperature control unit. Lignin solution was heated to 80° C. andcontinued heating at 80° C. for one hour. Then, the temperature of thelignin solution was decreased to 45° C.

In the second step, 342 g of phenol and 310 g of formalin (concentration52.5%) were added to the lignin solution. The temperature was increasedto 90° C. The reaction mixture was continuously heated at thetemperature at 90° C. for 110 minutes and then cooled down to roomtemperature. The reaction was monitored by measuring the viscosity usingthe Brookfield DV-II+LV viscometer.

The resin was analyzed and the results of the analysis are given inTable 4.

TABLE 4 Resin Properties S.C (%) 58 pH 8.8 Gel time (min) @100° C. 60

In view of the above detailed description of the present invention,other modifications and variations will become apparent to those skilledin the art. However, it should be apparent that such other modificationsand variations may be effected without departing from the spirit andscope of the invention.

1. A method for dissolving lignin in an aqueous medium comprising thesteps of a) mixing lignin, alkali and formaldehyde in an aqueous medium;b) mixing until a solution of lignin in the aqueous medium has beenobtained; wherein the molar ratio between the formaldehyde and ligninmonomer is 0.4 or less.
 2. A method according to claim 1, wherein themolar ratio between the formaldehyde and lignin is in the range of from0.01 to 0.3.
 3. A method according to claim 2, wherein the molar ratiobetween the formaldehyde and lignin is in the range of from 0.01 to 0.2.4. A method according to claim 1, wherein the lignin has been isolatedand/or purified before step a).
 5. A method according to claim 4,wherein the lignin has been isolated from black liquor before step a).6. A method according to claim 1, wherein the alkali is sodiumhydroxide, potassium hydroxide, calcium hydroxide, magnesium hydroxideor a mixture thereof.
 7. A method for preparing a resin comprising thesteps of a) mixing lignin, alkali and formaldehyde in an aqueous medium;b) mixing until a solution of lignin in the aqueous medium has beenobtained; c) keeping the mixture obtained in step b) at a temperature inthe range of from 30° C. to 95° C. and; d) adding additionalformaldehyde and phenol and e) maintaining the mixture at a temperatureof from 30° C. to 95° C. for at least 30 minutes; wherein the molarratio between the formaldehyde and lignin in step a) is 0.4 or less. 8.A method according to claim 7, wherein the ratio between theformaldehyde and lignin in step a) is from 0.01 to 0.3.
 9. A methodaccording to claim 7, wherein the ratio between the formaldehyde andlignin in step a) is from 0.01 to 0.2.
 10. A method for increasing thesolubility of lignin in an aqueous medium comprising the steps of a)mixing lignin, alkali and formaldehyde in an aqueous medium; b) mixinguntil a solution of lignin in the aqueous medium has been obtained;wherein the molar ratio between the formaldehyde and lignin is 0.4 orless.
 11. A method according to claim 10, wherein the ratio between theformaldehyde and lignin is in the range of from 0.01 to 0.3.
 12. Amethod according to claim 11, wherein the ratio between the formaldehydeand lignin is in the range of from 0.01 to 0.2.
 13. A resin obtainableby the method of claim
 10. 14. Laminate, engineered wood product orinsulation product manufactured using a resin according to claim
 13. 15.A method according to claim 4, wherein the lignin has been isolated fromblack liquor and purified before step a).